The following description is provided to assist the understanding of the reader. None of the information provided or references cited is admitted to be prior art.
Traditionally, a laser has three main components: an excitation source, an active medium, and a laser cavity. The excitation source is often an energy source such as light, heat, or current. The excitation source excites electrons in the active medium, and upon decay the electrons in the medium emit energy in the form of a light photon. An external cavity laser is a laser with optical elements external to the active medium that either form a laser cavity or form part of a laser cavity. A laser cavity serves as a resonator for the light photons in the cavity. Generally, a resonator is used in a laser to make the light emitted from a laser coherent. If a light beam from a laser is perfectly coherent, all the light will propagate in the same direction, will have the same wavelength, and will have the same phase. This is the ultimate goal of a laser's resonator, though perfect coherence is not generally achievable. In order to form coherent light, the intrinsic properties of a cavity allow only particular wavelengths to resonate in the cavity. If there are photons emitted by the active medium that align with the resonant traits of the cavity, those photons will not continue to propagate within the cavity. While the photons of the particular wavelength of the cavity (and the wavelengths of other integral multiples of the cavity) propagate, constructive interference of the photons boosts the amplitude of the waves within the cavity. The wavelengths emitted by a certain type of active medium are often a function of the type of active medium and the type and intensity of the excitation source. Thus, a laser cavity should be constructed to match the wavelengths emitted by the active medium in order for the laser to function properly.